Cooking vessels such as saucepans, frying pans, pans, pie pans, pots and the like are generally made of aluminum or its alloys and have their bottom as flat as possible to ensure complete contact with the heat sources. During heating, however, the bottom surface is not heated uniformly. Indeed, the bottom surface central zone is generally at a temperature relatively higher than that of the marginal zone. In addition, the lower face of the bottom in contact with the heat source is generally hotter than the top face. These temperature differences are the source of different expansion of the metal, reducing the life of the vessel.
A proposal to obviate these shortcomings, according to French patent No. 1,086,887 provides the bottom with concentric undulations or grooves which permit absorption of the deformations due to radial forces created by irregular expansion of the different parts of the bottom. One disadvantage of this type of bottom is that it is capable of absorbing from the heat source only the thermal energy transmitted by direct contact, i.e. the contact heat. The heat transmitted by radiation is reflected from the bright surface of the aluminum or its alloys and thus contributes in reduced measure to heating of the vessel. In addition, since there is a tendency to produce vessel bottoms as thin as possible to save material, very often the flat shape of the bottom is deformed after long use of the vessel. This causes a substantial drop in contact heat transmission.
Another proposal to overcome this shortcoming has been made in U.S. Pat. No. 4,926,843 which describes a process according to which a cooking vessel bottom is provided with concentric and spaced grooves coated with a layer of heat absorbent material only opposite to the grooves while this layer is removed from the surface of the bottom in the zones between the grooves, i.e. in the concentric lands. The heat absorbing material is generally the same enamel used to coat the outer surface of the vessel.
Known enamels designed for coating aluminum or its alloys have the drawback of being soft, i.e. presenting low abrasion and scoring resistance. This drawback is due to the composition of the frit of the enamel used, the cooking temperature of which must be lower than the melting or softening temperature of the aluminum or its alloys. For this reason coating with enamels is limited to the side or internal surfaces (grooves) of the vessel while the surfaces in relief of the bottom remain bare. This limitation of the enamelled surfaces causes incomplete radiation heat absorption.